I've seen the 120gb HD with transfer kit recently at Best Buy here in the States. I was shopping around, though, and was curious as to if there were any issues with it before making the jump.
I'm amazed you still have 11gb free on a 20gb HD!
I'm finding that I can't store more than 2 or 3 full demos without running out of capacity. I'm currently fluctuating in the 3-5 gb range. I do still have a lot of arcade trial games that I could probably get rid of, though.
One of the things eating up a lot of space are map packs (CoD3's Valor being the largest at over 800mb). And knowing that there's more to come in various games, I'm looking at the possibility of needing more space.
GH & RB songs are a lot larger than I originally believed they were as well. And there's many songs out there that I'd like to eventually DL when I have the chance to play them.
Waiting to bump up the HD capacity certainly couldn't hurt since I still have stuff that I don't use but could clean up. But like you suggest, availability at retail is a hit-or-miss.